This invention relates to the treatment of waste material, and, more particularly, to the controlled thermal destruction of hazardous and non-hazardous materials.
Placing waste material in landfills was previously the accepted method of disposal. When the consequences of landfill disposal were investigated more closely, public opposition and regulatory pressures restricted the landfill practice and compelled the industry to instead employ incineration, the only other then-available technology that was economical and appeared to address the disposal problems.
Incineration proved useful where landfill space was unavailable or too expensive, but, for a number of reasons, it also was generally inadequate. The basic nature of medical waste, for example, creates substantial problems for incinerators. One of the major problems encountered in using incinerators to combust medical waste is the heterogeneity of the waste material. This problem prevents the incinerators from maintaining a sufficiently high constant temperature to completely treat all of the organic and inorganic material in the waste, which can result in hazardous bottom or fly ash. For example, a first bag of such waste may be filled with containers of fluids, blood soaked bandages, and sharp objects (syringes, glass, metal surgical tools, and the like), while a second bag may contain mostly plastics, paper, packing material, pads, surgical gowns, rubber gloves, and the like. These two bags, fed independently into an incinerator, would create totally different combustion conditions. The first bag would quench and cool the combustion process, while the second bag would accelerate and raise temperatures.
During the low temperature cycle, products of incomplete combustion (pollutants) and potentially hazardous organic materials, such as dioxin, furan, and greenhouse gases, may be generated and ultimately released into the atmosphere. During the high temperature cycle, particulate, nitrogen oxide, and metal oxide emissions increase, including hexavalent chromium, a known carcinogen.
Shredding waste before feeding it into the combustion vessel can homogenize and mix the waste, but it may not be acceptable because of the potentially infectious nature of the waste and the inherent problem of disinfecting a shredder having numerous internal components and small confined places where infectious material might collect and escape disinfection. Moreover, some states may have laws prohibiting bags of infectious waste from being opened prior to their final processing.
Compounding the problem of temperature control within incinerators is the batch method of feeding that is commonly used (in contrast to continuous feeding). In this method, a ram system is normally used to push a charge of waste into a combustion vessel. Because the incinerator relies on the waste itself for fuel, as the waste combusts, vessel temperatures vary as the amount of combustible waste in the vessel changes. This problem is especially pronounced at start-up and shut-down. Temperatures also vary with changing feed rates and incinerators can operate poorly at reduced feed rates.
It can be important to achieve and maintain high temperatures because the treatment of inorganic waste components commonly found in medical and other waste streams requires such temperatures. High temperatures are required to melt stainless steel and borosilicate glass used in laboratories, for example, and incinerators may require fossil fuel additions to supplement the combustion process to reach these temperatures.
The destruction of organic waste also requires high temperatures, but instead of melting at high temperatures, such waste decomposes and burns if sufficient air is present. The combustion process can be self-sustaining only if enough heat energy is released during the process to cause additional material to decompose. This can be a problem in incinerators, however, and especially when wet and inorganic materials are present in the feed. Under such conditions, it may not be possible to maintain a high, continuous operating temperature.
Apparatuses that have used plasma torches to improve on the low and varying temperature problem have only achieved a partial solution. For example, a known ram (or batch) feed system causes significant variation in gas flow rates and temperatures, and includes no precautionary measures to hold the exit gas temperature at a safe high level at which reformation of complex organic compounds is minimized. The off-gas piping, for example, is composed of stainless steel and it leads to a steel cyclone for particulate collection, which causes the gas temperature to drop into a sufficiently low range (i.e., into the approximately 350–500° C. range). When the temperature of the gas drops to such temperatures, significant reformation of undesirable organic compounds, and particularly polycyclic aromatic hydrocarbons (PAH's), can occur.